The present invention relates generally to controlled current inverter drive systems for powering an AC motor load, and more particularly to means for preventing unacceptably high levels of motor current from occurring as a result of system response to abnormal operating conditions.
In the past, DC (direct current) motors have been used for operation over a wide speed range as desired. More recently, AC motors have been finding greater application in variable speed drive applications. This is due in a large measure to the fact that an AC induction motor for example is inherently rugged, thereby reducing maintenance problems due to the lack of brushes, which makes AC motors particularly desirable for certain applications. There are, however, certain problems associated with the use of AC motors, particularly when the motor is supplied by power from a variable frequency thyristor controlled inverter (DC to AC) which in turn is fed power from a thyristor controlled converter (AC to DC).
In the above cross-referenced related U.S. Pat. No. 4,230,979 and in a publication entitled, "A High Performance Controlled Current Inverter Drive," authored by Loren H. Walker, et al., a IEEE Conference paper and appearing in the IAS Annular Conference Record data Oct., 1979 at pages 927-936, both of which are specifically incorporated herein by reference, there is disclosed a controlled current inverter system comprised of a thyristor bridge configuraton for supplying an AC load such as an induction motor with an AC current of variable magnitude and variable frequency. The system described therein employs a variable DC power source which is connected to a variable frequency inverter preferably by way of a DC link including an inductor. Means are included to develop signals representing the instantaneous electrical torque of the AC motor and the instantaneous angle .theta. (defined as the angle between the air gap flux and the stator current) which is representative of the air gap power factor. The electrical torque signal and the angle signal are utilized to control the DC current in the link and the gating angle of the inverter thyristors with respect to the motor flux such that the air gap power factor or angle .theta. is controlled in a predetermined manner over its prescribed operating range.